The present invention relates to an apparatus and method for soldering electronic components to printed circuit boards.
Soldering of various electronic components are accomplished, for example, by dip soldering and reflow soldering. In the solder dip process, the leads of discrete components such as resistors and capacitors are inserted into holes in a printed circuit board, or surface mount components are glued onto one side of the printed circuit board with their leads contacted with pads. The board and the components are coated with flux. The flux operation is followed by a preheating operation wherein the flux coating is dried, and the board is preheated. The board and the electronic components are thereafter dipped into a molten solder bath to apply a molten solder to selected areas of the printed circuit board. The applied molten solder is cooled to solidify, thereby forming a solder joint. In the reflow soldering, a solder paste is applied to selected areas of a printed circuit board. The solder paste is typically composed of solder particles mixed with flux, adhesives, binders and other components. Surface mount components such as quad flat packs, small outline integrated circuits, capacitors and chip resistors are pressed against the applied solder paste. The adhesives hold the surface mount components to the printed circuit board. The printed circuit board is preheated in a preheat zone of a reflow solder oven. The printed circuit board is then passed through a reflow zone. This causes the solder particles in the solder paste to melt. The printed circuit board is finally transported to a cool down zone wherein the molten solder is cooled to solidify, thereby forming a solder joint.
In either process, fans are provided to rapidly cool the applied molten solder and solder paste, for example, at a rate of approximately 3.0xc2x0 C. or higher per second. In some cases, the cooling rate is even higher than 10xc2x0 C. per second in order to prevent the occurrence of xe2x80x9clift-offxe2x80x9d of a thin film of copper from a printed circuit board.
One very common type of solder composition used in electronics fabrication is a tin/lead alloy. The tin/lead alloy, being eutectic, has a melting point of approximately 183xc2x0 C. The temperature of the molten solder bath and the reflow solder oven are in a range of between approximately 220xc2x0 C. and 230xc2x0 C. Within this temperature range, printed circuit boards and electronic components are substantially free from thermal shock. The tinaead alloy has been selected and preferred because of superior wetting characteristics. The tin/lead alloy also yields highly reliable solder connections. However, the use of the tin/lead alloy in the fabrication of printed circuit boards is becoming more and more problematic due to the toxic effects of lead exposure to workers and the inevitable generation of hazardous waste. Thus, there is a great need to limit the amount of lead entering into the environment.
Compositions containing bismuth and indium are attempted as substitutes for the tinaead alloy. Such compositions can have a significantly low melting point, but are likely to cause discontinuities or fractures in solder connections on printed circuit boards. Other substitute compositions typically contain silver, copper, zinc, nickel, chromium, molybdenum, iron, cobalt, phosphorus, germanium and/or gallium. All of these compositions have relatively high melting points as low as 200xc2x0 C., for example. Thus, the use of any of these compositions results in a significant increase in the temperatures of a molten solder bath, for example, as low as 250xc2x0 C., and a reflow oven, for example, as low as 240xc2x0 C. The time of exposure of printed circuit boards to such elevated temperatures must be avoided to prevent thermal shock to the boards. For this reason, applied molten solder and solder paste are conventionally cooled at a rate of at least 3.0xc2x0 C. per second.
Where the molten solder is rapidly cooled, a portion of the molten solder which solidifies in an early stage is not subject to a change in volume. On the other hand, a portion of the molten solder which solidifies in a final stage is subject to substantial contraction. This results in the formation of cavities and other defects in solder connections. Also, when the printed circuit board is rapidly cooled, there arises a difference in temperature between the outer surface of the electronic components and the inside of the electronic components. The surface of the electronic components are thus subject to contraction. This contraction creates fractures and cracks in solder connections.
Accordingly, it is an object of the present invention to provide an apparatus and method for soldering electronic components to printed circuit boards which prevents the occurrence of fractures and cavities in solder connections and damage to electronic components to be soldered.
According to one aspect of the present invention, there is provided an apparatus for soldering electronic components to a substrate which comprises a conveyor for transporting the substrate along a predetermined path, a fluxer located below the conveyor for coating the substrate and the electronic components with a flux, a preheater located below the conveyor and downstream of the fluxer for heating the substrate and the electronic components to a predetermined temperature, a supply of molten solder located below the conveyor and downstream of the preheater for applying a molten solder to selected areas of the substrate, and a cooling assembly arranged downstream of and adjacent to the supply of molten solder. The cooling assembly is operable to gradually cool the applied molten solder at a rate of less than or equal to approximately 1.0xc2x0 C. per second until the molten solder reaches its solidus temperature.
In a preferred embodiment, the cooling assembly may comprise at least one infrared heating element for directing heated air over the substrate, a housing within which the infrared heating element is arranged, and a porous metal plate mounted on the open top of the housing and contacted with the infrared heating element. The porous metal plate may be provided at its top surface with a ceramic layer. Preferably, a shroud may be communicated with the housing and include a fan for drawing ambient air into the shroud and feeding the air to the housing. This arrangement enables hot air to be circulated continuously through the cooling assembly.
As an alternative, the cooling assembly may comprise a plurality of infrared heating elements arranged within a housing and adapted to apply infrared heat over the substrate. Still alternatively, the cooling assembly may comprise an elongated enclosure extending upstream toward the fluxer and terminating at one end of the preheater adjacent to the fluxer. The enclosure may be shaped to define a space over the preheater, where the supply of molten solder and the cooling assembly are to confine heat dissipated from the preheater and the supply of molten solder within the space.
According to another aspect of the present invention, there is provided a reflow solder oven for soldering electronic components to a substrate, which comprises a conveyor for transporting the substrate along a predetermined path, a preheat zone for heating the substrate to which a solder paste is applied, a reflow zone located adjacent to and downstream of the preheat zone for melting solder particles in the solder paste, and a cool down zone adjacent to and downstream of the reflow zone for cooling the melted solder particles at a rate of less than or equal to approximately 1.0xc2x0 C. per second until the melted solder particles reach their solidus temperature.
In a preferred embodiment, the cool down zone may comprise a pair of top and bottom heater assemblies for directing heated air over the substrate.
According to a further aspect of the present invention, there is provided a method for soldering electronic components to a substrate, which comprises transporting the substrate along a predetermined. path, coating the substrate and the electronic components with a flux, preheating the substrate and the electronic components to a predetermined temperature, applying a molten solder to selected areas of the substrate, and gradually cooling the applied molten solder at a rate of less than or equal to approximately 1.0xc2x0 C. per second until the molten solder reaches its solidus temperature.
In a preferred embodiment, heated air may be directed over the substrate to cool the molten solder. Alternatively, infrared radiation may be applied to the substrate to cool the molten solder.
According to a still further aspect of the present invention, there is provided a method of soldering electronic components to a substrate, which comprises applying a solder paste to selected areas of the substrate, transporting the substrate along a predetermined path, preheating the substrate to a predetermined temperature, melting solder particles in the solder paste, and gradually cooling the melted solder particles at a rate of less than of equal to approximately 1.0xc2x0 C. per second until the melted solder particles reach their solidus temperature.
The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.